Sizing dispersions

ABSTRACT

An aqueous anionic sizing dispersion which comprises an anionic dispersion of a cellulose-reactive sizing agent and colloidal anionic aluminum modified silica-based particles. The dispersion is used in the production of paper, board or cardboard whereby the dispersion is either added to the stock or applied as a surface size.

The present invention relates to anionic sizing dispersions and moreparticularly to such dispersions which comprise an anionic dispersion ofa cellulose-reactive sizing agent and colloidal anionicaluminum-modified silica particles.

Aqueous dispersions of cellulose-reactive sizing agents, like alkylketene dimer (AKD) and alkenyl succinic anhydride (ASA), are widely usedfor sizing at neutral or slightly alkaline pH levels in order to impartliquid repellent or hydrophobic properties to cellulose-based products.Anionically stabilized dispersions are usually used in stock sizingapplications involving high levels of cationic charge, for example as aresult of cationic papermaking aids added to the stock, where retentionof cationically stabilized dispersions has proved problematic. Suchhighly cationic wet-end systems are used for instance in the productionof liquid packaging board and for photographic paper.

Commercial anionic AKD dispersions are sufficiently stable to permittransportation, storage and usage. In order to maximize shelf life andsizing efficiency, the dispersions are kept at low temperatures, withoutbeing allowed to freeze, and the products usually have a shelf life ofat least one month at room temperature. However, there have beenexamples of agglomeration and separation of storage-stable anionic AKDdispersions when subjected to high shear forces, in particular atpumping and mixing operations. The agglomeration and separation hasresulted in deposition problems, an increased need of maintenance anddifficulty in dosing the dispersion.

It is known from EP 564994 to incorporate alum in sizing compositions ofcationic ketene dimers and cationic colloidal silica, alumina orzirconia in order to aid in stabilization.

The object of these compositions is to achieve the desired level ofsizing while increasing the coefficient of friction of the papersurface.

EP 418 015 discloses sizing compositions comprising a ketene dimer andan anionic dispersing agent. In addition, an extender or modifyer suchas an anionic polyacrylamide, an anionic starch or colloidal silica canbe present to extend or modify the anionic charge density of thecompositions.

According to the present invention it has been found that the stabilityof anionic dispersions of cellulose-reactive sizing agents can beimproved by the incorporation of colloidal anionic aluminum-modifiedsilica particles. The anionic sizing dispersions according to theinvention have less tendency to agglomerate and form deposits whensubjected to shear forces and show very good shelf life at storagetemperatures usually employed. In comparison with anionic sizingdispersions containing colloidal silica particles, the presentdispersions comprising colloidal anionic particles of aluminum-modifiedsilica show improved stability. Introducing aluminum-modified silicaparticles into the anionic sizing dispersions also results in a verygood sizing with low amounts of the sizing agent when used in stocksystems where high levels of cationic charge are evident, for exampleresulting from cationic wet-strength resins, cationic starches,polyacrylamides and polyethyleneimines added to the stock, since a verygood retention of the sizing agent is obtained.

The present invention thus relates to aqueous anionic sizing dispersionsas further defined in the claims.

The cellulose-reactive sizing agent which is present in the dispersionsaccording to the invention can be selected from any of thecellulose-reactive sizing agents known in the art. Suitably, thecellulose-reactive sizing agent is selected from the group consisting ofhydrophobic ketene dimers, acid anhydrides, organic isocyanates,carbamoyl chlorides and mixtures thereof, preferably ketene dimers andacid anhydrides, most preferably ketene dimers.

Suitable ketene dimers have the formula (I) shown below, in which R¹ andR² represent hydrocarbon groups suitably having from 8 to 36 carbonatoms, usually being alkyl groups having 12 to 20 carbon atoms, such ashexadecyl and octadecyl groups. Suitable acid anhydrides can becharacterized by the general formula (II) shown below, in which R³ andR⁴ can be identical or different and represent saturated or unsaturatedhydrocarbon groups suitably containing from 8 to 30 carbon atoms, or R³and R⁴ together with the --C--O--C-- moiety can form a 5 to 6 memberedring, which can be substituted with hydrocarbon groups containing up to30 carbon atoms. Examples of acid anhydrides which are used commerciallyinclude alkyl and alkenyl succinic anhydrides (ASA) and particularlyisooctadecenyl succinic anhydride. ##STR1##

Suitable ketene dimers, acid anhydrides and organic isocyanates includethe compounds disclosed in U.S. Pat. No. 4,522,686, which is herebyincorporated herein by reference. Examples of suitable carbamoylchlorides include those disclosed in U.S. Pat. No. 3,887,427 which isalso incorporated herein by reference.

The sizing dispersions according to the invention comprise colloidalanionic aluminum-modified silica particles. The term colloidal anionicaluminum-modified silica particles used herein is meant to includecolloidal anionic particles having at least a surface layer of aluminumsilicate, including pure aluminum silicates, and colloidal anionicparticles of silicic acid or silica which are surface-modified withaluminum. It is preferred that the particles are surface-modified withaluminum. Colloidal anionic particles of the above-mentioned types whichcan be present in the the dispersions according to the invention areknown in the art, for example from U.S. Pat. No. 4,961,825, U.S. Pat.No. 4,980,025, U.S. Pat. No. 5,368,833 and WO 94/05596, which are herebyincorporated herein by reference. The colloidal particles are usuallycontained in aqueous sols. The aluminum-modified particles are suitablysurface-modified with aluminum to a degree of 2 to 25% and preferablyfrom 3 to 20%, and hereby is meant the number of aluminum atoms whichhas replaced silicon atoms in the particle surface. The degree ofmodification is given in % and is calculated on basis of 8 silanolgroups per nm², as described by Iler, R. K. in Journal of Colloidal andInterface Science, 55(1976):1, 25-34. The colloidal anionicaluminum-modified silica particles can have a size less than about 500nm and the size is usually greater than 1.5 nm, corresponding to aspecific surface area in the range of about 5 to about 1800 m² /g.Suitably, the particle size is in the range of from about 2.5 to about270 nm, corresponding to a specific surface area in the range of fromabout 10 to about 1000 m² /g. Preferably, the specific surface area ofthe particles is from 30 to 950 m² /g. The specific surface area can bemeasured by means of titration with NaOH in conventional manner, forexample according to the method described by Sears in AnalyticalChemistry 28(1956):12, 1981-1983.

Anionic dispersions according to the invention can have contents ofcellulose-reactive sizing agents from about 0.1% by weight up to about30% by weight. The content of cellulose-reactive sizing agent issuitably within the range of from 5 to 25% and preferably from 8 to 20%by weight.

In the dispersions according to the invention the weight ratio ofcellulose-reactive sizing agent to aluminum-modified silica particlescan be within the range of from 1:1 to 100:1. The weight ratio issuitably within the range 1.5:1 to 30:1 and preferably within the range2:1 to 20:1. The solids content of the dispersions usually exceeds 1% byweight and can reach 50% by weight. The solids content suitably exceeds5% by weight. The upper limit is suitably 40% and preferably 30% byweight.

The dispersions according to the present invention may contain one orseveral anionic dispersing agents in amounts which are sufficient toconfer the desired negative charge to the dispersions. Suitably, theamount of dispersing agent is at least 0.5% by weight, based on theamount of sizing agent. Normally, it is not necessary to use more than10% by weight.

The dispersing agents can be any of those conventionally used at thepreparation of aqueous anionic sizing dispersions or emulsions. They canfor example be selected from saponified rosin derivatives, alkylsulphates, alkylaryl sulphates, alkyl sulphonates, alkylarylsulphonates, etc. Particularly suitable anionic dispersing agents arealkyl sulphates and alkyl sulphonates, e.g. sodium lauryl sulphate, aswell as sodium lignosulphonate and sodium naphthalene sulphonate.

If desired, non-ionic, anionic, amphoteric or cationic protectivecolloids and non-ionic, amphoteric or cationic dispersing agents may beincluded in the dispersions, preferably in minor amounts and providedthat the overall charge of the total amount of dispersing agents andoptional protective colloids which are present in the dispersions isnegative or anionic. Such compounds may advantageously be included indisersions of higher dry contents. As examples of suitable protectivecolloids can be mentioned water-soluble cellulose-derivatives such ashydroxyethyl- and hydroxypropyl-, methylydroxypropyl- andethylhydroxyethylcellulose, methyl- and carboxymethylcellulose, gelatin,starch, guar gum, xanthan gum, polyvinyl alcohol, etc.. Non-ionicdispersing agents can for example be selected from ethoxylated fattyalcohols, fatty acids, alkyl phenols or fatty acid amides, ethoxylatedor non-ethoxylated glycerol esters, sorbitan esters of fatty acids, etc.Suitable cationic dispersing agents and protective colloids can forexample be selected from nitrogen-containing compounds such asquaternary ammonium compounds, salts of tertiary amines, water-solublenitrogen-containing epichlorohydrin resins and cationic starches, etc..The dispersion may also contain other additives such as preservativeagents.

The dispersions according to the invention can for example be preparedby simply mixing an anionic dispersion or emulsion of acellulose-reactive sizing agent with a sol of colloidal anionicaluminum-modified silica particles, such as any of those describedabove. Anionic dispersions of cellulose-reactive sizing agents and solsof colloidal anionic aluminum-modified silica particles are known in theart and commercially available.

The anionic dispersions according to the invention can be used in aconventional manner in the production of cellulose-based products,including paper, board and cardboard. They can be used both for surfacesizing and internal or stock sizing at the production of such products.The present invention also relates to a method for the production ofcellulose-based products using an aqueous anionic dispersion containinga cellulose-reactive sizing agent and colloidal anionicaluminum-modified silica particles, as defined above, as surface orstock sizing agents. The method is of course advantageous to thepapermaker in that the sizing dispersion has high sizing efficiency andimproved stability which reduces the tendency of the sizing agent toform deposits and thus simplifies high shearing operations such aspumping and dosing. Suitably, the amount of cellulose-reactive sizingagent either added to the stock containing cellulose fibers, andoptional fillers, or applied on the cellulose-based product as a surfacesize, usually at the size press, is from 0.01 to 1.0%. by weight, basedon the dry weight of cellulose fibers and optional fillers, preferablyfrom 0.05 to 0.5% by weight, where the dosage is mainly dependent on thequality of the pulp or cellulose-based product to be sized, thecellulose-reactive sizing agent used and the level of sizing desired.

Chemicals conventionally added to the stock in paper-making such asretention aids, aluminum compounds, dyes, wet-strength resins, opticalbrightening agents, etc., can of course be used in conjunction with thepresent dispersions. Examples of suitable retention aids includecationic polymers, bentonite in combination with cationic polymers andsilica-based sols in combination with cationic polymers or cationic andanionic polymers. Examples of aluminium compounds include alum,aluminates and polyaluminium compounds such as polyaluminium chloridesand sulphates.

Particularly good stock sizing can be obtained when using thedispersions of the invention in combination with cationic polymers. Asexamples of suitable cationic polymers can be mentioned cationic starch,guar gum, polyacrylamide, polyethyleneimine, polyamidoamine andpoly(diallyldimethyl ammoniumchloride) and combinations thereof.Cationic starch and polyacrylamide are preferably used, either alone orin combination. When use is made of stocks containing high levels ofso-called anionic trash, the addition to the stock of cationic polymersas mentioned above can be preceded by the addition of a low molecularweight cationic polymer in order to reduce the cationic demand of thestock. The present dispersions can be added before, between, after orsimultaneously with the addition of the cationic polymer or polymers.

The invention is further illustrated in the following examples, which,however, are not intended to limit the same. Parts and % relate to partsby weight and % by weight, respectively, unless otherwise stated.

EXAMPLE 1

Anionic sizing dispersions were prepared by mixing an anionic ketenedimer dispersion with various sols of anionic silica-based particles.The following sols were used:

Sol A: A sol of colloidal anionic particles of aluminium-modifiedsilicic acid having a specific surface area of 500 m² /g and beingaluminum-modified to a degree of 9%, as disclosed in U.S. Pat. No.4,961,825.

Sol B: A sol of colloidal anionic aluminium-modified silica particleshaving a specific surface area of 220 m² /g and being aluminum-modifiedto a degree of 5%, commercially available under the tradename Bindzil305/220, Eka Nobel.

Sol C: A sol of colloidal silica particles having a specific surfacearea of 500 m² /g, as disclosed in U.S. Pat. No. 4,388,150.

Sols A and B were used for preparing dispersions according to theinvention whereas sol C was used for preparing dispersions forcomparison purposes. In order to prepare the dispersions, the sols werediluted with water to a SiO₂ concentration of 15% by weight, acidifiedwith H₂ SO₄ to a pH of 3.5 and the obtained acid sols were mixed with ananionic ketene dimer dispersion (hereinafter designated Ref. AKD) havingan AKD content of 9.6%, solids content of 10.7% and pH of 3.5, inamounts of 10 and 20% by weight (calculated as SiO₂ on dry ketenedimer), respectively. The following sizing dispersions were prepared:A10, A20, B10 and B20 according to the invention, and Ref. C10 and Ref.C20 for comparison purposes, the capitals A, B, C and FIGS. 10, 20referring to the silica-based sol used and the amount thereof,respectively.

EXAMPLE 2

Stability of the anionic dispersions according to Example 1 wasevaluated by means of an aggregation and deposition test, a so-calledglass tube test, in which the dispersions after storage at 25° C. for2-4 weeks were subjected to high shear forces for a set time before theevaluation.

In the glass tube test, the dispersion was placed in a glass tube whichwas sealed and then vigorously shaken for 15 minutes. The glass tube wasinspected visually and the amount of deposition formed on the interiorwall of the tube was correlated to the stability of the dispersion,which was expressed as good, fair or poor.

In addition, stability was tested by measuring the viscosity of theanionic dispersions according to Example 1 after storage at 40° C. for2-4 weeks. The viscosity was measured with a Brookfield RTV viscosimeter(spindle no. 2, 50 rpm).

The results are set forth in the table below.

    ______________________________________    Dispersion             Storage     Stability   Viscosity    used     (weeks)     Glass Tube Test                                     (cps) 40° C.    ______________________________________    Ref. AKD 2           fair        15    Ref. AKD 4           poor        16    Ref. C10 2           fair        15    Ref. C10 4           fair        16    Ref. C20 2           fair        20    Ref. C20 4           poor        72    A10      2           good        16    A10      4           good        16    A20      2           good        17    A20      4           good        16    B10      2           good        17    B10      4           good        16    B20      2           good        16    B20      4           good        18    ______________________________________

As is evident from the table, the dispersions according to the inventioncontaining colloidal anionic aluminum-modified silica particles showedimproved stability as compared to the anionic ketene dimer dispersionfrom which they were prepared and the anionic ketene dimer dispersioncontaining colloidal silica.

EXAMPLE 3

The sizing efficiency of a dispersion according to invention wasevaluated in this example. The dispersion was prepared in a mannersimilar to Example 1 from a sol having an S-value of 25% and containingcolloidal anionic silica particles having a specific surface area of 900m² /g which were surface-modified with aluminum to a degree of 5%, asdisclosed in U.S. Pat. No. 4,961,825, and the anionic ketene dimerdispersion, Ref. AKD, used in Example 1. The content of silica-basedparticles was 30% by weight, calculated as SiO₂ on dry ketene dimer. Theobtained sizing dispersion, designated D30, showed good stability afterstorage at 25° C. for one week, evaluated by means of the glass tubetest described in Example 2.

Paper sheets were prepared from a standard stock of bleached sulphatepulp (60% birch and 40% pine) at a pH of 7.5 according to the standardmethod SCAN-C23X for laboratory scale. The dispersion Ref. AKD was usedfor comparison purposes. The dispersions were added to the stock inamounts of 0.5, 1.0 and 2.0 kg/tonne, respectively, calculated as ketenedimer sizing agent on dry cellulose fibres, followed by the addition of6 kg/tonne of cationic potato starch. The obtained Cobb-values, measuredaccording to Tappi standard T 441 OS-63, are shown in the table below.

    ______________________________________    Dispersion     AKD dosage                             Cobb 60    used           (kp/tonne)                             (g/m.sup.2)    ______________________________________    Ref. AKD       0.5       29.0    Ref. AKD       1.0       22.0    Ref. AKD       2.0       20.5    D30            0.5       19.5    D30            1.0       18.5    D30            2.0       17.0    ______________________________________

As can be seen, the dispersion according to the invention showed veryhigh sizing efficiency.

I claim:
 1. Aqueous anionic sizing dispersion which comprises an anionicdispersion of a cellulose-reactive sizing agent selected from the groupconsisting of hydrophobic ketene dimers; acid anhydrides, organicisocyanates, carbamoyl chlorides and mixtures thereof, and colloidalanionic aluminum-modified silica particles wherein the ratio ofcellulose-reactive sizing agent and aluminum-modified silica particlesis in the range of from 1:1 to 100:1, and the solids content of thedispersion is from 1 to 50% by weight.
 2. Sizing dispersion according toclaim 1 wherein the weight ratio of cellulose-reactive sizing agent toaluminum-modified silica particles is in the range of from 2:1 to 20:1.3. Sizing dispersion according to claim 1 wherein the aluminum-modifiedsilica particles have a size less than 500 mn.
 4. Sizing dispersionaccording to claim 1 wherein the aluminum-modified silica particles havea specific surface area within the range of from 10 to 1000 m² /g. 5.Sizing dispersion according to claim 1 wherein the cellulose-reactivesizing agent is present in an amount of from 0.1% by weight to 30% byweight.
 6. Sizing dispersion according to claim 1 wherein thecellulose-reactive sizing agent is an alkyl ketene dimer.
 7. Sizingdispersion according to claim 1 wherein the aluminum-modified silicaparticles are selected from the group consisting of particles having atleast a surface layer of aluminum silicate and silica particles whichare surface modified with aluminum.
 8. The sizing dispersion of claim 1wherein the cellulose reactive sizing agent is a hydrophobic acidanhydride.
 9. The sizing dispersion of claim 1 which comprises at leaseone anionic dispersing agent.
 10. The sizing dispersion of claim 9 whichcomprises at lease 0.5% by weight anionic dispersing agent based on theamount of sizing agent.
 11. The sizing dispersion of claim 1 whereinsaid ketene dimer is of the formula: ##STR2## wherein R¹ and R² arehydrocarbon groups having from 8-36 carbon atoms.
 12. The sizingdispersion of claim 1 wherein the sizing agent is present in an amountof from 5 to 25% by weight.
 13. A process for the production ofcellulose-based products which comprises adding to said cellulose-basedproduct, an aqueous anionic sizing dispersion which comprises an anionicdispersion of a cellulose-reactive sizing agent selected from the groupconsisting of hydrophobic ketene dimers; acid anhydrides, organicisocyanates, carbamoyl chlorides and mixtures thereof, and colloidalanionic aluminum-modified silica particles wherein the ratio ofcellulose-reactive sizing agent and aluminum-modified silica particlesis in the range of from 1:1 to 100:1, and the solids content of thedispersion is from 1 to 50% by weight.
 14. A process according to claim13, wherein the cellulose-based products comprise paper, board andcardboard.
 15. The process of claim 13 wherein said aqueous sizingdispersion is either added to the cellulose stock or is applied as asurface size.
 16. The process of claim 15 wherein the aqueous sizingdispersion is added to the stock in combination with a cationic polymerselected from cationic starch, guar gum, polyacrylamide,polyethyleneimine, polyamidoamine and poly(diallyldimethylammoniumchloride) and combinations thereof.